In order to limit engineering costs and to achieve purchasing savings through repetitiveness, standardised ranges of air separation apparatus have been created, ranging up to tonnages of around 700 Mt/day, or even 1000 Mt/day. These standardised productions do not always exactly correspond to the requirements of the customer or customers in terms of output and/or pressure but the cost on these small units is the main optimisation factor, and standardisation responds well to this key criterion.
Beyond these capacities, since the energy assumes greater and greater importance, so-called modular units have been introduced, the orientation this time being to standardise certain key parts, but to follow the requirements of the customers as closely as possible and to take into account in the sizing the parallel constraints of energy and investment.
EP-A-0504029 describes a pump cycle based on the concept of monomachine with a single large high-pressure air compressor.
This approach provides appreciable savings in investment compared with a traditional pump cycle, by introducing all the energy necessary with this single air machine, the discharge pressure of which may be between approximately 12 bara and 35 bara, whatever the purities and pressures of the productions required. However, this single machine, when we arrive at very high powers, is difficult to implement and starts with complex and expensive starting artifices at the motors, referred to as regulators. The number of manufacturers is also extremely small, which limits, without however destroying it, the technical and economic advantage of this approach. Some of these problems are described in “Turbomachinery Limitations for Large Air Separation Plants” by Wolentarski, Cryogenic Processes and Equipment Conference, Century 2—Emerging Technology Conferences, San Francisco, Calif., Aug. 19-21, 1980.
For reasons of maintenance and reliability, spare parts are purchased for all these critical machines, with regard to both the compressors and the motors. It is entirely acceptable to have a single set of spare parts for a group of identical machines installed on the same site, or even in the same country.
Depending on the power, the technology of the motors varies: in fact beyond 25 MW, there is on the market no motor other than synchronous, the current technology of asynchronous motors not making it possible to go beyond this limit without taking very great industrial risk.
The article “Oxygen Plants: 10 years of development and operation” in CEP July 1979 describes the use of synchronous motors and explains that three sizes of synchronous motor are stored for replacing the European compressors of the Air Liquide group, in the event of breakdown.
In general terms, the equipment cost of an air separation unit with cycles with a single high-pressure air compressor (apart from storage and vaporisation vessels and high-voltage utilities) breaks down into four main parts:
i) Compression function (compression, motor, starting equipment and associated electrics): 45% to 50%.
ii) Cold box function and associated equipment: 30% to 35%.
iii) Purification function for the hot part of the air before entry into the cold box: 10% to 15%.
iv) Miscellaneous: 5% to 10%.
It is therefore clear that reducing costs and increasing the reliability of the compressors, motors and starting equipment is a priority.
With methods using a cold booster driven by a turbine, as described in U.S. Pat. No. 5,475,870, or the methods as described in EP-A-0504029, all the power is introduced by the high-pressure air compressor. A booster is a compressor that compresses a gas from a pressure higher than atmospheric pressure. It is also possible to compress all the air at high pressure and not to use a booster or to use only boosters coupled to an air and nitrogen turbine, as in EP-A-0504029, so that all the power is introduced by a single high-pressure air compressor. The arrangements at the heat-exchange line, the number and type of turbines coupled to a booster and the distillation columns makes the productions compatible with the purities, pressures and throughputs required by the customer.